1. Field of the Invention
Embodiments of the invention generally relate to methods for activating a conductive material and depositing a capping layer on a semiconductor feature, and more particularly, at least in one embodiment, methods for depositing a silver activation layer on a copper surface for subsequent deposition of a cobalt alloy layer.
2. Description of the Related Art
Recent improvements in circuitry of ultra-large scale integration (ULSI) on substrates indicate that future generations of semiconductor devices will require smaller multi-level metallization with smaller geometric dimensions. The multilevel interconnects that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio features, including contacts, vias, lines and other features. Reliable formation of these interconnect features is very important to the success of ULSI and to the continued effort to increase circuit density and quality on individual substrates and die as features decrease in size.
Currently, copper and its alloys have become the metals of choice for sub-micron interconnect technology because copper has a lower resistivity than aluminum, (e.g., 1.67 μΩ-cm for Cu and 3.1 μΩ-cm for aluminum at room temperature), a higher current carrying capacity, and significantly higher electromigration resistance. These characteristics are important for supporting the higher current densities experienced at high levels of integration and increased device speed. Further, copper has a good thermal conductivity and is available in a highly pure state.
The use of copper as a conductive material poses several challenges. Copper readily migrates under current and may contaminate the dielectric material by way of diffusion. Also, copper forms copper oxide when exposed to atmospheric conditions or environments outside of processing equipment and therefore requires passivation to prevent copper oxide formation. Copper oxides increase the resistance of the copper layer, become a source of particle contamination and reduce the reliability of the overall circuit.
One solution to minimize electromigration and oxidation of copper is to deposit a capping layer on the exposed copper surface. Cobalt alloys have been observed as suitable materials for capping copper surfaces and are generally deposited by electroless deposition techniques. However, copper does not easily catalyze or initiate the deposition of materials with lower reduction potential, such as cobalt or nickel, from conventional electroless solutions. One approach to initiate cobalt alloy deposition on a copper surface is to apply a reducing current to initiate deposition through a galvanic reaction. However, one difficulty of using a reducing current to initiate cobalt alloy deposition is the availability of a continuous conductive surface over the substrate surface. A conductive surface may not be available with many applications, particularly after planarization of the copper and dielectric layers by chemical mechanical polishing (CMP) techniques.
Another approach to activate the copper surface is to deposit a catalytic material on the copper surface prior to depositing a cobalt alloy layer. However, deposition of the catalytic material may require multiple steps or use catalytic colloid compounds, such as palladium colloids. Catalytic colloid compounds may adhere on dielectric materials and result in undesired, excessive and non-selective deposition of the cobalt alloy materials on the substrate surface. Non-selective deposition of cobalt alloy materials may lead to surface contamination, unwanted diffusion of conductive materials into dielectric materials, and even device failure from short circuits and other device irregularities.
Another approach to catalyze or initiate deposition of cobalt alloys is to deposit a more electropositive metal on the copper surface. A catalytic activation layer may be deposited on the copper layer and is generally composed of a single, noble metal, such as palladium or platinum. The noble metal activation layer may be deposited on the copper layer to provide good adhesion between the copper layer and the capping layer. However, the noble metal/copper interphase, such as a palladium/copper (Pd/Cu) interphase, has a higher resistivity than copper (i.e., an increase of about 1 μΩ-cm for 1% atomic Pd in Cu over pure Cu). The higher resistivity reduces current and is not a desirable trait. Also, noble metal deposition solutions generally require a high metal ion (e.g., Pd2+ or Pt2+) concentration in order to obtain deposited layers. The deposition processes utilizing palladium or platinum are usually not efficient at plating or reducing the metal ions onto the copper surface and therefore much of the expensive and unconsumed metal ions are discarded as waste when the depleted deposition solution is discarded. Furthermore, while platinum and palladium are often used as activation materials due to their high catalytic properties, platinum and palladium are often not easily deposited selectively on the conductive layer and contaminate the dielectric layer of a substrate surface.
Although cobalt alloys are deposited as capping layers to protect copper layers from destructive oxidation, cobalt alloys are susceptible to oxidation which forms metal alloyed oxides. Once the cobalt alloy is oxidized, there is a likelihood the copper underlayer may also be oxidized due to oxygen passage from the metal alloyed oxides to the copper layer. Metal oxides in the capping layer or on the copper layer are undesirable because of the increased resistivity.
Therefore, there is a need for a method to form a semiconductor feature including a capping layer with low electrical resistance, strong barrier properties and good adhesion to a conductive layer.